The present invention relates to blood pressure measurement, in particular to a method and apparatus for the indirect measurement of blood pressure.
In most automatic indirect methods of blood pressure measurement, a pressure cuff is attached to a patient's arm adjacent a blood vessel, the cuff is pressurized with an applied pressure which is high enough to occlude the blood vessel and the applied pressure is gradually reduced. As the pressure is reduced to below systole and then diastole, blood begins to flow through the blood vessel creating the well known Korotkoff sounds and pulsatile pressures (oscillations) in the blood vessel. The sounds can be detected by a microphone and the pressure oscillations by a pressure transducer. The sensor, whether a microphone or pressure transducer, measures a quantity which is representative of the patient's blood pressure.
A table is then formed of values of the quantity measured at various applied pressures (oscillometric values) as the applied pressure is gradually changed. Using the table the systolic and diastolic blood pressures are determined. In some blood pressure measurement systems where the applied pressure is bled down in steps, at each step the peak amplitude of the oscillations is detected. By measuring the time elapsing between successive peaks the heart rate can be calculated. A table can be formed of the calculated heart rate versus applied pressure.
In a well behaved reading of blood pressure, the oscillometric values generally increase from low values at applied pressures above the systole to a maximum value at applied pressures between systole and diastole. Similarly, the values generally increase from low values at applied pressures less than diastole to the maximum values. In some cases, however, spurious high or low values in the table can occur due to patient movement, etc., which can and often do cause erroneous determinations of systolic and diastolic blood pressures. It is desirable therefore to detect and eliminate the spurious high and low values in the table when they occur, and to use either or both of the above mentioned tables (pressure and heart rate) to reject measurements where a high level of suspected artifact is present.